Some of the most overlooked diseases around the world are parasitic infections, especially those caused by helminths such as hookworms and schistosomes. These worms, which live inside their host, are especially common in developing countries. As we hear from health reporter Rose Hoban, a new study found that if you're infected with one, you're probably infected with others, too.

Brown University researcher Amara Ezeamama was studying the frequency of individual parasitic diseases, when it occurred to her that many people are probably infected with more than one parasite at a time. So she set out to study how common a phenomenon that was. She studied several hundred children from villages in the Philippines.

Ezeamama was shocked by what she found. "In our data, people were infected by 2, 3, 4 parasites over 93% of the time," she reports. "You know, being free of infection or having only one infection happened only 7% of the time in the entire study."

After further study, Ezeamama realized that it was to be expected. "Most of these infections are soil-transmitted infections and they get contracted through a lot of hand-to-mouth activity," she explains. "Or in the case of hookworm boring through the skin if you come in contact with infected soil."

Ezeamama says that parasitic infections are diseases of poverty. Millions – if not billions – of people who have multiple parasites are constantly exposed to them in their environment. She says the same factors that expose people to parasites are the same circumstances that force poor people to live in places that have these infections.

"So there's a lot of co-linearity, meaning that if you contract one, it's likely you've contracted another because they live in similar places," Ezeamama says. "Some point in their life cycle [is] in the soil… kids are walking around without shoes and a lot of things, it's not surprising to me just how people end up with a lot of them at once."

But what Ezeamama did find surprising was that being infected with one parasite made people more likely to be infected with another. And having more than one parasite elevated the risk of anemia in children.

"It is likely that you will have anemia, if you have any of these infections, even without the other," Ezeamama says. "However, when you have two of them together, the risk that you experience is not just hookworm plus Schisto, it's much more. It's about three times what we would have expected if those infections had independent effects."

Ezeamama says it's vital that health officials in poor countries systematically address both the parasitic infections and the conditions that lead to children being infected with different kinds of parasites at once.

Her research is published in the online journal PLoS Neglected Tropical Diseases.